1. Field of the Invention
The invention relates to hydrocarbon processing. Specifically, the invention relates to upgrading of unstable naphthas, such as coker naphthas, to produce high quality distillate and motor fuel.
For years, poor quality or hard to process gasoline boiling range streams such as coker, visbreaker or pyrolysis naphtha, have been a problem for refiners.
These materials contain such high quantities of di-olefins, in addition to sulfur and nitrogen compounds, that they are extremely difficult to process in conventional refinery units. The large di-olefin content of such streams renders them extremely reactive or unstable. If an attempt is made to simply hydrotreat these streams in a conventional hydrotreater, the reactive di-olefins form gum which plugs the conventional hydrotreating bed, or less frequently plugs the heat exchanger or heater upstream of the hydrotreating unit.
These unstable naphthas are of such poor quality that they cannot be blended into the refinery gasoline pool. Refiners have resorted to some rather extreme steps in order to deal with these materials.
High pressure naphthas are of such poor quality that they cannot be blended into the refinery gasoline pool. Refiners have resorted to some rather extreme steps in order to deal with these materials.
High pressure hydrotreating, at reactor pressures of 1000-1500 psig, is one way to handle the problem. Coker naphthas, either alone, or with other naphtha boiling range stocks, are treated with a conventional hydrotreating catalyst (such as Co-Mo or Ni-Mo on a support such as alumina) in a hydrotreating reactor operating with an extremely high hydrogen partial pressure. This approach works, and reduces gum formation to tolerable levels, but high pressure hydrotreating is expensive, and not usually required for naphtha boiling range streams. A refiner having a source or coker naphtha must build a separate high pressure hydrotreater to handle the coker naphtha. If the coker naphtha is blended with conventional gasoline boiling range materials and hydrotreated, then the blend must be processed at high pressure. This means the hydrotreater must be a very large one, operating at high pressure.
Catalytic di-olefin conversion upstream of conventional hydrotreating is also possible. Such proprietary technology is available from various licensors. UOP, Inc. has offered Platfining process, which operates at very low temperatures, temperatures low enough so that gum formation does not occur. The catalyst is reportedly active enough even at these low temperatures to convert the di-olefins in the feed to something else. The product of such processes can then be co-mingled with other naphtha streams for conventional hydrotreating, reforming, etc. The drawback to this approach is that it requires an extra processing unit for the unstable naphtha stream upstream of the conventional naphtha upgrading processes.
Fluid catalytic cracking (FCC) processing of unstable, naphtha boiling range materials was reported in U.S. Pat. No. 4,051,013, which is incorporated herein by reference. The patentee used a hot, clean burned FCC catalyst to first contact an unstable naphtha fraction in a riser, with addition of gas oil feed in a downstream portion of the riser. The patentee reported that 1000 BPD of coker naphtha could be converted in this manner into 510 BPD of FCC gasoline, with the remainder being converted to coke (5 wt. %) and C4.sup.- (42 wt. %).
Hydroformylation of fluid coker naphtha was reported in U.S. Pat. No. 4,711,968, which is incorporated herein by reference. Using a special catalyst system, comprising a soluble rhodium or cobalt carbonyl complex catalyst, the patentee was able to achieve hydroformylation of many olefin containing streams. Although the hydroformylation process could proceed without prior purification, the patentee suggested various feed treatment steps, e.g., removal of mercaptans by extraction, and removal of sulfur, as well as nitrogen compounds, by adsorption on columns packed with polar solids such as silica, fuller's earth, bauxite. The use of zeolites to enrich the feeds in 1-n-olefins and n-paraffins was taught. Removal of aromatic compounds by selective solvent extraction was taught. Sulfur compounds could also be removed by passing cracked distillate through a high temperature fixed bed of bauxite, fuller's earth or clay.
Upgrading of pyrolysis gasoline from steam cracking to make ethylene, by passing the naphtha over Pd/Zn/ZSM-5 at 900 to 1200 F was disclosed in U.S. Pat. No. 4,097,367, which is incorporated by reference. The high temperature processing of the C.sub.5 + fraction converted to aromatics everything boiling in the BTX range, yielding a liquid product with essentially no non-aromatic hydrocarbons boiling above 167.degree. F. The patentee also discussed the general prejudice in these arts re. hydrogen, namely that ZSM-5 is known for conversion of olefins to aromatics, but preferably in the absence of hydrogen.
None of the above approaches provided a complete solution to the problem of upgrading unstable naphtha streams. High pressure hydrotreating is expensive and produces a low value product. Removal of di-olefins by selective catalysis adds a fairly expensive processing step which yields as a product a low value naphtha stream. The hydroformylation approach does not meet the needs of refiners who want to make gasoline and distillate.
None of the prior art processes provide a way to efficiently convert an unstable naphtha feed into more valuable liquid fuel components, comprising gasoline and distillate components. In the FCC processing of coker naphtha, only about half of the coker naphtha, by weight, is converted to a normally liquid product, with the rest converted to coke or light gases. Many refineries lack eitner the facilities, or the market, for production of more light material and for these reasons FCC processing of coker naphtha is not a good upgrading method.
We have discovered a way to efficiently upgrade these unstable naphthas to high octane gasoline and distillate boiling range components in a simple process which can operate for a long time at very mild operating conditions.